Optical encoders are used in a wide variety of contexts to determine movement and/or a position of an object with respect to some reference. Optical encoding is often used in mechanical systems as an inexpensive and reliable way to measure and track motion among moving components. For instance, printers, scanners, photocopiers, fax machines, plotters, and other imaging systems often use optical encoding to track the movement of an image media, such as paper, as an image is printed on the media or an image is scanned from the media.
One common technique for optical encoding uses an optical sensor and an encoder code scale (or encoding media). The optical sensor focuses on a surface of the code scale. As the sensor moves with respect to the code scale (or encoding media), or the code scale moves with respect to the sensor, the sensor reads a pattern of light either transmitted through, or reflected by, the code scale to detect the motion.
A typical code scale is an alternating series of features. As the encoder and sensor move relative to the one another, transitions from one feature to the next in the pattern are optically detected. For instance, a code scale could be an alternating pattern of holes, or optically transmissive windows, in an opaque material. In which case, an optical sensor can detect transitions from darkness to light passing through the holes or windows.
FIG. 1 illustrates a basic optical encoder 100 comprising an optical unit 103 including an optical emitter 101 and an optical sensor 102, and a light controlling member (optical code scale) 105 disposed between the optical emitter 101 and the optical sensor 102. Optical unit 103 and optical code scale 105 can move relative to each other in a linear fashion longitudinally of optical code scale 105. FIG. 2 is another view of optical encoder 100.
In one common application, optical unit 103 is mounted on the printing head of a printer, optical code scale 105 is fixed to a case of the printer, and optical unit 103 moves along the length of code scale 105 when the printing head moves. As optical unit 103 moves along the length of optical code scale 105, light from optical emitter 101 passing through optical code scale 105 is sensed by optical sensor 102 to produce one or more signals that indicate the relative movement between optical unit 103 and optical code scale 105. The output signal or signals from optical sensor 102 are then used by the printer to help control the movement of the printing head and/or paper in the printing process.
However, there are some problems that can occur to the optical sensor when the optical encoder is installed in a printer as described above. For example, ink aerosol is a problem which can shorten the lifetime of optical encoder 100. In particular, during the printing process ink particles can become airborne and then deposited on optical emitter 101, optical code scale 105, and/or optical sensor 102. When that happens, the light path from optical emitter 101, to code scale 105, and finally to optical sensor 102, is partially or completely blocked by the ink particle(s) so that optical encoder 100 does not operate properly. In that case, maintenance or replacement of all or part of the optical encoder 100 is required. Undesirably, maintenance or replacement of optical encoder 100 can be expensive and decreases the availability of the host device (e.g., printer).
What is needed, therefore, is an optical sensor capable of remedying a malfunction due to the deposition of one or more ink particles on an optical encoder, and an optical encoder including such an optical sensor.